Rotational mask scanning exposure method and apparatus

ABSTRACT

A method and rotational mask scanning apparatus for exposing a plurality of images on a workpiece, include a rotatable mask having a pattern of image segments thereon, an optical system for projecting the image segments onto the workpiece, and a device for at least one of rotating the mask and for moving the workpiece so as to continuously expose a plurality of regions on the workpiece with the pattern of image segments.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to increasing thethroughput of scanning lithography systems, and more particularly to arotational mask scanning method and apparatus for increasing thethroughput of such systems.

[0003] 2. Description of the Related Art

[0004] Overlay and image size specifications continue to decrease at arapid pace driving the migration from traditional step and repeatexposure tools to scanning systems which have improved opticalsubsystems. However, while the improved optics of the scanning systemsmay meet the more rigorous specifications, the throughput of thesesystems is nonoptimal which increases the operation costs.

[0005] Conventional translational scanning masks utilize considerableamounts of time stopping and starting. After each field is printed, areticle of a scanning system must be stopped, repositioned, andaccelerated to the selected scanning speed. As a result, considerablemass reduction design constraints are placed on the reticle and reticlestage.

[0006] Thus, even though the traditional translational masks utilizevery high acceleration/deceleration and scanning speeds to maximize thetime spent exposing the wafer, considerable time is still lost instopping and starting, as mentioned above. As a result, this greatlyreduces the throughput of the scanning systems and increases thecomplexity and cost of the reticle stage.

[0007] Furthermore, fundamental throughput limits are being reached dueto the increasing percentage of time spent on overhead tasks (e.g.,stopping, starting, aligning, etc.) as opposed to time spent exposingthe wafer. The conventional systems do not allow for future throughputimprovements as wafer stage speed and laser power increases.

SUMMARY OF THE INVENTION

[0008] In view of the foregoing and other problems of the conventionalmethods and systems, an object of the present invention is to provide astructure and method for improving throughput and for simplifyingreticle handling.

[0009] Yet another object is to provide for continuous scanning across awafer without having to stop or start the mask or wafer.

[0010] A still further object is to optimize efficiency by designing therotational mask size to match the printed field size.

[0011] In a first aspect of the present invention, a photo-exposure toolfor exposing a plurality of images on a workpiece, includes a rotatablemask having a pattern of image segments thereon, an optical system forprojecting the image segments onto the workpiece, and a device for atleast one of rotating the mask and for moving the workpiece so as tocontinuously expose a plurality of regions on the workpiece with thepattern of image segments.

[0012] Thus, with the inventive method and system, throughput isoptimized and operation costs are reduced as compared to theconventional methods and systems.

[0013] Moreover, the present invention provides a rotational mask whichallows for continuous printing of fields without constant starting andstopping. Therefore, higher overall throughput, flexibility in design,and less frequent acceleration/deceleration are achieved.

[0014] Furthermore, the inventive structure allows for future throughputimprovements as wafer stage speed and laser power increases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing and other objects, aspects and advantages will bebetter understood from the following detailed description of preferredembodiments of the invention with reference to the drawings, in which:

[0016]FIG. 1 is a flow diagram illustrating a preferred method offorming a mask according to the present invention;

[0017] FIGS. 2A-2C are schematic diagrams of an optical system (e.g., ascanning exposure system) according to a first preferred embodiment ofthe present invention; and

[0018] FIGS. 3A-3B are schematic diagrams of an optical system (e.g., ascanning exposure system) according to a second preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0019] Referring now to the drawings, and more particularly to FIGS.1-3B, preferred embodiments of the present invention will be describedhereafter.

[0020] Generally, in a first preferred embodiment, a rotational mask isemployed in a scanning exposure tool to improve throughput and simplifyreticle handling. Maximum efficiency can be achieved by designing therotational mask size to match the printed field size, thereby allowingfor continuous scanning across the wafer without needing to stop orstart the mask or wafer. Fields are printed one after the other in arow. Printing in the opposite direction requires that the rotationaldirection of the mask be reversed. If rows are printed in only one scandirection, then the mask rotation can remain constant.

[0021] A second preferred embodiment allows the mask circumference to belarger than the printed field size. In this approach, fields are notprinted continuously. At the completion of each field, the reticle andwafer stage must be repositioned to the following field byacceleration/deceleration of one or both components.

[0022] Process of Forming Mask

[0023] Prior to examining the preferred system structures, anillustrative process 100 for constructing a cylindrical mask for anoptical system (e.g., a scanning exposure system) is described hereinbelow and by referring to FIG. 1. The mask is preferably cylindrical orpart of a cylinder. For example, the mask could be formed as a segmentportion of a cylinder. For maximum efficiency, the mask should becurved.

[0024] First, as shown in step 101, a starting material is provided. Forexample, a polished cylinder may be provided, made of at least one ofquartz, silica or another material transparent at the exposingwavelength (e.g., the wavelength of a light source employed). Someexemplary cylindrical dimensions include a radius of within the range ofabout 10 mm to about 100 mm, and preferably about 25 mm, a thicknesssubstantially within a range of about 2 mm to about 10 mm, andpreferably about 6 mm, and a length substantially within a range ofabout 20 mm to about 300 mm, and more preferably about 200 mm. Thedimensions (especially width and length) of the mask are selected tooptimize a scanning exposure of a selected workpiece. Obviously, themask's dimensions may be selectively changed.

[0025] Thereafter, the processing of the starting material begins.

[0026] As shown in step 102, the cylinder is cleaned preferably using asolvent such as, for example, acetone, two-proponal or aqueous solutionsof hydrogen peroxide and water, sulfuric acid/hydrogen peroxide mix, andthe like. Other cleaning methods also can be employed, as are known inthe art, such as sputter cleaning with an inert gas such as argon, orplasma “ashing” with oxygen or ultraviolet (UV) “ashing” with ozone.

[0027] Then, as shown in step 103, the ends of the cylinder are enclosedor capped with a material that is insensitive to subsequent processing.For example, candidate capping materials may include quartz, Teflon, orother rigid or semi-rigid materials that can be used in a vacuum and areresistant to wet chemicals such as solvents, acids, oxidizers, or bases.

[0028] Thereafter, as shown in step 104, the cylinder is processeddepending upon the type of mask to be formed.

[0029] For example, for a reflective-type mask, the cylinder ispreferably solid and made of a material that can be polished to opticalflatness.

[0030] If a transparent material is used as the underlying substrate,the transparent material must first be coated with a thin film that willreflect the wavelength of the incoming light source. Candidate materialsfor the reflective thin film to be coated on the transparent materialinclude at least one of chrome, gold, aluminum, refractory metals, metalalloys or nitrides of refractory metals, and the like, all of which canbe deposited using conventional application techniques includingchemical-vapor deposition (CVD), sputter deposition, dip coating, orplating. Preferably, the thickness of the thin film coating deposited ona transparent substrate is substantially within a range of about 500 toabout 2000 Angstroms, and more preferably about 1000 Angstroms. Moregenerally, the coating is preferably thick enough to be uniformlyreflective and defect-free (e.g., pinholes, etc.).

[0031] For a transmission-type of cylindrical mask, the outer surface ofthe cylinder is coated with an opaque or semi-transparent material suchas at least one of gold, chrome, tungsten, titanium, tantalum, and otherrefractory materials, alloys thereof, as well as nitrides of tungsten,titanium, tantalum and other refractory metals. The coating is performedusing conventional application techniques including chemical-vapordeposition (CVD), sputter deposition, reactive sputter deposition, dipcoating or plating.

[0032] Additionally, compound or multi-layer materials can be used tocontrol reflections within the starting material substrate. For example,a plurality of layers may be formed such as a titanium layer on top of atitanium nitride layer which in turn is formed on a surface of thesubstrate. The thickness of the individual coating(s) preferably may besubstantially in a range of about 500 to about 2000 Angstroms each.

[0033] For a reflective-type mask, the entire outer surface of thecylinder (e.g., preferably only the surface to be rotated and opposed tothe workpiece) is coated with a material that will absorb the incomingwavelength. For example, such a material may include at least one of ananti-reflective chrome, silicon, carbon, nitrides of refractory metalssuch as titanium nitride, and such a material may be deposited bysimilar methods described above (e.g., CVD, sputtering, plating,reactive sputtering, etc.)

[0034] In some cases, the layer will absorb at a particular thicknessand is related to the refractive index of the deposited material and thewavelength of the incident light. For example, silicon thicker than 2000Angstroms will absorb UV light having a wavelength of 248 nm. Otherthin, transparent films will appear opaque at a given thickness due totheir refractive index and the wavelength of the incident light.

[0035] In preparation for patterning the opaque, semi-transparent, orabsorbing material, in step 105, the cylinder is coated with an adhesionpromoter such as hexamethyldisilazane (HMDS), which iscommonly-available.

[0036] For vacuum integrity, the end caps or enclosure are preferablyremoved at this point.

[0037] Then, in step 106, an electron-beam sensitive photoresist (e.g.,polybutylsulphone (PBS)) is applied to the outer periphery of thecylinder by conventional techniques such as roller-coating,spin-coating, or spray-coating.

[0038] As shown in step 107 of FIG. 1, in preparation for electron-beam(e.g., e-beam) exposure, the coated cylinder is baked in a convectionoven for approximately 30 minutes at a temperature preferablysubstantially in a range of about 120° C. to about 150° C., and morepreferably at about 130° C.

[0039] Then, in step 108, the cylinder is exposed using an electron-beamtool which “writes” the desired pattern in the e-beam resist (e.g., seeU.S. Pat. No. 5,269,882, incorporated herein by reference, whichdescribes a method and process for writing a pattern on a resist coatedcylinder).

[0040] Thereafter, the end caps are preferably placed back on thecylinder during the wet and/or dry processing.

[0041] Then, as shown in step 109, the pattern on the cylinder isdeveloped, thereby removing the resist from areas that were not exposedby the e-beam tool. The cylinder is again baked in a conventional ovenfor about 30 minutes at about 150° C., for example.

[0042] Then, in step 110, the cylinder is etched by exposing thecylinder to an appropriate etching agent. For example, the agent may bea wet chemical such as an acid or a base that preferentially etches theopaque material (e.g., chrome) not covered by the e-beam resist.Alternatively, the agent may be a dry process material such as a plasmaetch, thereby to preferentially etch the areas for the cylinder that arenot covered by the e-beam resist.

[0043] Thereafter, in step 111, the remaining e-beam resist is removedfrom the cylinder using an appropriate wet or dry process. For example,a wet process could be a sulfuric acid/hydrogen peroxide mixture whichremoves the e-beam resist oxygen-rich plasma.

[0044] Then, in step 112, preferably the end caps are removed, and themask (e.g., cylinder) is completed and now ready for use in the exposuretool. The exposure tool may be a scanning exposure system using arotational mask, as described below and as shown in FIGS. 2A-3C.

[0045] First Preferred Embodiment

[0046] Referring to FIGS. 2A-2B, a scanning exposure system 20 is shownwhich incorporates a rotational mask as built by the exemplary andnon-limiting process above. The rotational mask preferably is atransmission mask having a transparent surface such that the light isilluminated from within the mask along opaque images formed on thesurface of the mask.

[0047] As shown in FIG. 2A, the system 20 includes a rotational mask 21having a substrate 21 a on which a plurality of opaque images (e.g.,patterns) 21 a are formed on the mask's outer periphery. Additionally oralternatively, a plurality of interference patterns may be formed on thesurface of the mask. A light source 22 is preferably formed such thatlight is emitted through the rotational mask. For example, the lightsource could be formed in the interior of the mask, thereby forming anillumination system. The illumination system also could include avariety of other systems. Alternatively, the light source could beformed external to the mask with suitable light reflecting means, suchas reflecting mirrors and the like. With such an external light sourceusing the reflecting mirrors, the light beam may be brought into themask and reflected through the mask.

[0048] The rotational mask 21 may be a large cylinder or a smallcylinder of any practical length and radius depending upon the size ofthe image to be formed on the object. There is no fundamental limit tothe size of the mask. Further, as mentioned above, the mask can be asegment (portion) of a cylinder.

[0049] The mask 21 modulates light 22 a output by the light source 22.The light may be modulated by any type of light modulator such as aslit, a pin-hole, an iris, a shutter, etc. The modulated light 22 a isinput to a lens system 23.

[0050] The lens system 23 may be a reduction lens system to imagefine/small patterns on the object to be imaged. Alternatively, the lenssystem can be a magnifying system which magnifies the image for aparticular application if desired. The lens system 23 may be a singlelens or a plurality of lenses depending upon the designer's application,constraints and design requirements. The same lens system can be used asin the first embodiment. The object 24 (workpiece) to be imaged receivesthe light from the lens system 23.

[0051] The rotational mask 21 is rotated by a rotary device 25. Device25 includes conventional means well known in the art, such as a highaccuracy and precision stepping motor.

[0052]FIG. 2B illustrates a cross-section of mask 21 (e.g., preferably acylindrical mask) formed having the transparent substrate 21 a. A seriesof opaque patterns 21 b are formed on the substrate 21 a, thereby toform an image on the object 24 to be imaged. As discussed above in thefirst embodiment, the light source 23 may be built into the mask, oralternatively the light source is preferably external to the mask and areflecting mirror or the like can be used to input light to thetransparent mask and the then the light is reflected through thetransparent mask. Typically, just the outer periphery of the mask withthe opaque patterns rotate. The light source may be equipped with alight modulator 23 a such as a slit, pin-hole, an iris, etc. The maskhas a radius 21 c which may be within a range substantially betweenabout 10 mm and about 100 mm, depending upon the application.

[0053] In the first preferred embodiment, the rotational mask employedin the scanning exposure tool improves throughput and simplifies reticlehandling. By designing the rotational mask size to match the printedfield size (e.g., printed on the object to be imaged), continuousscanning across the wafer can be achieved without needing to stop orstart the mask or wafer. Fields are printed one after the other in arow. The workpiece is moved continuously, and the source does not move.Printing in the opposite direction requires that the rotationaldirection of the mask be reversed. If rows are printed in only one scandirection, then the mask rotation can remain constant.

[0054] With the scanning system shown in FIGS. 2A-2B including arotational mask 21, continuous printing of fields is made possiblewithout constant starting and stopping of the mask and wafer. Therefore,higher overall throughput, flexibility in design, and less frequentacceleration/deceleration are achieved.

[0055] Another advantage is that repeating images can be printed withouta seam. For example, for liquid crystal displays (LCDs) today, manyimages are printed together to make one display. That is, a field sizeof the LCD is smaller than the overall LCD display. Thus, a plurality ofimages (or image portions) must be formed and “assembled” together toform the overall image. However, the position where the images (or imageportions) overlap creates problems. Such problems do not arise with thepresent invention since no seam is present, even when repeating imagesare printed. Many applications would find great benefit from thisfeature, including, for example, the printing of repetitive fabricpatterns, etc.

[0056] Further, in LCD displays, scans can be performed a plurality oftimes (e.g., two or more) with many revolutions per scan. Thus, multiplerotations may be formed by rotating the rotational mask a plurality oftime (e.g., past 360 degrees). Hence, the invention is capable ofprinting beyond 360 degrees.

[0057] Second Preferred Embodiment

[0058] Turning to FIGS. 3A-3C, a second preferred embodiment of thescanning system 30 is shown which includes a reflective, rotational mask31. The system 30 is substantially similar to the first embodiment shownin FIGS. 2A-2C but for the reflective mask 31 and using an externallight source 32.

[0059] As shown in FIG. 3A, the mask is preferably cylindrical andincludes a reflective layer 31 a formed on its outer periphery. Further,the mask 31 includes opaque image 31 b on top of the reflective surface.A light source/illuminator 32 is shown external to the mask 32, therebyforming an illumination system. The illumination system could be avariety of systems (e.g. optical, e-beam, x-ray, etc.)

[0060] The rotational mask 31 is rotated by conventional means wellknown in the art and as described above as rotary device 25. Therotational mask 32 may be a large cylinder or a small cylinder of anypractical length and radius depending upon the size of the image to beformed on the object. Preferably, the size of the area on the object tobe imaged substantially corresponds to the size of the rotational mask.

[0061] The mask 31 modulates light 32 a output by the light source 32.Specifically, the light is modulated by the opaque portions and/orinterference patterns formed on the mask 32. As before in the firstembodiment, the modulated light 32 a is input to a lens system 33.

[0062] The lens system 33 may be as above with the first embodiment, andmay be a reduction lens system to image fine/small patterns on theobject to be imaged. Alternatively, the lens system can be a magnifyingsystem which magnifies the image for a particular application ifdesired. The lens system 33 may be a single lens or a plurality oflenses depending upon the designer's application, constraints and designrequirements. The same lens system can be used for the second embodimentas in the first embodiment. The object 34 (workpiece) to be imaged maybe mounted on a stage 35 for easy and precise positioning and handlingof the object 34. A stage also may be used in the first embodiment.Generally, the stage is a standard component known in the art, and thestage can be designed for lower cost and higher throughput of thesystem.

[0063]FIG. 3B illustrates a cross-section of mask 31 (e.g., preferably acylindrical mask) formed having a substrate 36 having a reflective layer31 a formed thereon. A series of opaque patterns 31 b are formed on thereflective layer 31 a, thereby to form an image on an object to beimaged.

[0064]FIG. 3C illustrates a perspective view of the cylindrical mask 31.As shown in FIG. 4B, the mask 31 includes a plurality (e.g., two in theexemplary arrangement of FIG. 3C) of opaque patterns on a field ofreflective material 31 a.

[0065] With the scanning system shown in FIGS. 3A-3B including arotational mask 31, continuous printing of fields is made possiblewithout constant starting and stopping. Therefore, higher overallthroughput, flexibility in design, and less frequentacceleration/deceleration are achieved. Hence, the second embodiment hasthe same capabilities as the first embodiment, and can print continuousor sequential images.

[0066] Furthermore, utilizing existing illumination sources and waferstages, there is a significant improvement in throughput of the system.Another advantage is that this design obtains significant benefit by anyincrease in laser power or stage velocity, whereas existing, flat masksystems obtain minimal benefit. Instead, the flat masks must rely on amuch more expensive implementation to increase acceleration/decelerationof wafer and reticle stages.

[0067] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

[0068] As would be appreciated by one of ordinary skill in the artwithin the purview of the present application, the present invention hasapplications other semiconductor wafer imaging. For example, theinvention would find great benefit in applications and areas whererepetitive images are printed at very high resolution. Computer cardprinting and liquid crystal display (LCD) devices are but two examples.

What is claimed is:
 1. A photo-exposure tool for exposing a plurality ofimages on a workpiece, comprising: a rotatable mask having a pattern ofimage segments thereon; an optical system for projecting the imagesegments onto the workpiece; and means for at least one of rotating saidmask and for moving the workpiece so as to continuously expose aplurality of regions on the workpiece with said pattern of imagesegments.
 2. The tool according to claim 1 , wherein said mask comprisesa cylindrical mask.
 3. The tool according to claim 1 , wherein saidimages comprise identical images.
 4. The tool according to claim 1 ,wherein said mask includes a substrate, and wherein said patterncomprises a plurality of opaque portions formed on said substrate. 5.The tool according to claim 1 , wherein said optical system comprises alight source for transmitting a light beam through said mask.
 6. Thetool according to claim 1 , wherein said optical system comprises alight source for producing a light beam reflected from said mask.
 7. Ascanning exposure tool for producing images on an object having aphotoimageable film thereon, comprising: a light source for producing alight beam; a rotating mask having the light beam transmittedtherethrough; and a lens system for receiving said light beam from saidmask, wherein said light source is modulated by said rotating mask andprojected through said lens system to the object.
 8. The tool accordingto claim 7 , wherein said mask comprises a cylindrical mask.
 9. The toolaccording to claim 7 , wherein said mask includes a substrate, andincludes a pattern comprising at least one of a plurality of opaqueportions and a plurality of interference portions formed on saidsubstrate.
 10. A scanning exposure system for producing an image on anobject having a photoimageable film thereon, comprising: a rotating maskhaving a thin film reflective coating and at least one of an opaqueimage and an interference pattern formed on the reflective coating; alight source for directing light to said mask, said mask reflecting thelight to produce reflected light; and a lens system for receiving thereflected light from said mask.
 11. The system according to claim 10 ,wherein the light directed from said light source to said mask ismodulated by the at least one of the opaque image and the interferencepattern and the reflective coating of the mask, and is projected throughsaid lens system to the object.
 12. The system according to claim 11 ,wherein said mask comprises a cylindrical mask.
 13. The system accordingto claim 11 , wherein said mask includes a substrate, and a pattern ofimage segments formed thereon, and wherein said pattern comprises the atleast one of the opaque image and the interference pattern formed onsaid substrate.
 14. A method of exposing a plurality of images on aworkpiece, comprising: providing a rotatable mask having a pattern ofimage segments thereon; projecting the image segments onto theworkpiece; and rotating the mask and moving the workpiece so as tocontinuously expose a plurality of regions on the workpiece with saidpattern of image segments.
 15. The method according to claim 14 ,wherein said mask comprises a cylindrical mask.
 16. The method accordingto claim 14 , wherein said mask includes a substrate, and wherein saidpattern comprises a plurality of opaque portions formed on saidsubstrate.
 17. The method according to claim 14 , wherein saidprojecting the image segments onto the workpiece comprises transmittinga light beam through said mask.
 18. The method according to claim 14 ,wherein said projecting the image segments onto the workpiece comprisesreflecting a light beam from said mask.
 19. The method according toclaim 14 , wherein said images comprise identical images.
 20. The methodaccording to claim 14 , wherein said mask comprises a cylindrical mask,and said mask includes a substrate, and wherein said pattern comprises aplurality of opaque portions formed on said substrate, said imagescomprising identical images.
 21. The method according to claim 20 ,wherein said projecting the image segments onto the workpiece comprisestransmitting a light beam through said mask.
 22. The method according toclaim 20 , wherein said projecting the image segments onto the workpiececomprises reflecting a light beam from said mask.